In conventional credit/debit card and calling card applications there are only two calls involved. These are the inbound calls from the caller/subscriber into the service provider's platform, and the outbound call to the destination number. Computing the time remaining, based upon the available balance in the subscriber's account, is straightforward for the two call scenario even if both legs are being charged. However, modern calling systems can be much more complex. By way of example, there can be applications involving one number with multiple simultaneous inbound calls, multiple "follow-me" calls, or conferencing. Moreover, there can be more than two calls in progress at the same time.
Therefore, there is a need for a system of rating and debiting complex debit card calls. Also, there is a need for a system that coordinates call rating and debiting functions for all calls for a given subscriber to ensure that the charges incurred do not exceed the available balance in the subscriber's account. And finally, there is a need for these capabilities in real time.